1. Technical Field
The present invention relates to rotors for rotating electric machines that are used in, for example, motor vehicles as electric motors and electric generators.
2. Description of Related Art
There are known IPM (Interior Permanent Magnet) motors that have permanent magnets embedded in a rotor core. The IPM motors can use both reluctance torque and magnetic torque, thereby achieving high efficiency. Therefore, the IPM motors are particularly suitable for use in hybrid and electric vehicles.
An IPM motor generally includes a stator and a rotor that is disposed in radial opposition to the stator. The rotor includes a rotor core and a plurality of permanent magnets. The rotor core has a plurality of pairs of magnet-receiving holes formed therein. Each pair of the magnet-receiving holes is arranged in a substantially V-shape that opens toward the stator side. Each of the permanent magnets is received in a corresponding one of the magnet-receiving holes of the rotor core. Further, for each pair of the magnet-receiving holes, the two corresponding permanent magnets which are respectively received in the two magnet-receiving holes of the pair are arranged so as to together form one magnetic pole of the rotor. In addition, for each pair of the magnet-receiving holes, there is formed a corresponding center bridge that extends in a radial direction of the rotor core between the two magnet-receiving holes of the pair to separate them from each other.
Further, to reduce magnetic flux leakage through the corresponding center bridge, there is disclosed, for example in Japanese Patent Application Publication No. JP2006311730A (to be simply referred to as Patent Document 1 hereinafter), a technique of suitably designing magnetic flux barriers (or magnetic voids) at ends of the permanent magnets. More specifically, according to the technique, for each of the permanent magnets, there is provided a first magnetic flux barrier at a non-magnetic pole center-side end of the permanent magnet and a second magnetic flux barrier at a magnetic pole center-side end of the permanent magnet.
Moreover, according to the disclosure of Patent Document 1, each of the permanent magnets is arranged in the corresponding one of the magnet-receiving holes of the rotor core so that there is formed a gap between a radially outer side face of the permanent magnet and a radially outer wall surface of the corresponding magnet-receiving hole. The gap has a constant width at a circumferential central portion of the permanent magnet. Further, the gap has a greater width at circumferential end portions of the permanent magnet than at the circumferential central portion. Consequently, with the above arrangement of the permanent magnets in the corresponding magnet-receiving holes of the rotor core, it is possible to prevent local stress concentration from occurring in the rotor core.
In addition, according to the disclosure of Patent Document 1, for each of the permanent magnets, there are provided in the rotor core a pair of supporting portions that respectively support radially inner end portions of the first magnetic flux barrier-side and second magnetic flux barrier-side faces of the permanent magnet.
On the other hand, Japanese Patent Application Publication No. JP2011211860A (to be simply referred to as Patent Document 2 hereinafter) discloses a technique of providing relatively large second magnetic flux barriers each of which extends radially inward from a magnetic pole center-side end of a corresponding one of the magnet-receiving holes of the rotor core, thereby reducing magnetic flux leakage radially inward.
Moreover, Patent Document 2 also discloses that in the rotor core, there are provided, for each of the permanent magnets, a pair of supporting portions that respectively support radially inner end portions of the first magnetic flux barrier-side and second magnetic flux barrier-side faces of the permanent magnet. In addition, in a root part of the supporting portion that supports the radially inner end portion of the second magnetic flux barrier-side face of the permanent magnet, there is formed a void space with a predetermined length, so as to improve the operating efficiency of the rotating electric machine.
However, in operation of an IPM motor as described above, the permanent magnets generate heat with rotation of the rotor, causing the temperature of the rotor core to increase. Accordingly, with repetition of start and stop of the operation of the IPM motor, thermal stress will be repeatedly induced in the rotor core.
In particular, according to Patent Documents 1 and 2, for each of the permanent magnets received in the corresponding magnet-receiving holes of the rotor core, the first magnetic flux barrier-side and second magnetic flux barrier-side faces of the permanent magnet are respectively supported by the pair of supporting portions of the rotor core. Consequently, an excessive load due to thermal stress will be imposed on the pair of supporting portions, thereby making it difficult to ensure high strength of the rotor core.
Moreover, according to Patent Document 1, for each of the permanent magnets, the gap formed between the radially outer side face of the permanent magnet and the radially outer wall surface of the corresponding magnet-receiving hole of the rotor core has a greater width at the circumferential end portions of the permanent magnet than at the circumferential central portion. Consequently, it is particularly easy for demagnetization (or reduction of magnetic flux) to occur at both a magnetic flux loop that is created by magnetic flux leaking through the corresponding center bridge and completed within the rotor and a magnetic flux loop that is created by excitation by the stator of the rotating electric machine and passes between adjacent magnetic poles of the rotor.
On the other hand, according to Patent Document 2, for each of the permanent magnets, there is formed the void space in the root part of the supporting portion that supports the radially inner end portion of the second magnetic flux barrier-side face of the permanent magnet. Consequently, it is particularly easy for demagnetization to occur around a corner portion of the permanent magnet between the second magnetic flux barrier-side face and the radially inner side face of the permanent magnet.